The present invention relates to a magnetic memory device constructed as a magnetic random access memory (MRAM) or a so-called non-volatile MRAM (Magnetic Random Access Memory) which is comprised of memory elements made by laminating a magnetization pinned layer in which the orientation of magnetization is fixed and a magnetic layer in which the orientation of magnetization is changeable, or to any magnetic memory device comprising memory elements having a magnetic layer capable of being magnetized.
With a rapid prevalence of information communication equipment, in particular, of personal compact equipment such as portable terminals, a further improvement in performance inclusive of larger scale integration, faster speed, lower power consumption or the like is demanded for their elements such as memories, logics and the like.
In particular, the non-volatile memory is considered to be indispensable in the age of ubiquitous. If a power supply is exhausted or a power trouble occurs, or even if a connection between the server and the network is cut off by any failure, the non-volatile memory can protect important information including personal information. Further, although recent portable equipment is designed to hold unnecessary circuitry blocks in a standby state in order to suppress power consumption as much as possible, if a non-volatile memory that can function both as a high speed work memory and as a large capacity storage memory is realized, losses of the power consumption and the memory can be eliminated. Still further, if a high speed and large capacity non-volatile memory can be realized, an “instant-on” function that can be instantaneously activated upon power-on operation becomes possible.
As the non-volatile memories, there are also cited a flash memory which uses semiconductors, a FRAM (Ferroelectric Random Access Memory) which uses a ferroelectric material, and the like.
However, the flash memory has a drawback that a write speed is low in the order of μ seconds. On the other hand, also in the FRAM, problems are cited that because the number of rewritable frequencies is 1012 to 1014, the endurance thereof is too small to completely replace SRAM (Static Random Access Memory) or DRAM (Dynamic Random Access Memory), and that micro fabrication of a ferroelectric capacitor is not easy.
Drawing attention as a non-volatile memory having no such drawbacks as described above, featuring a high speed, large capacity (large scale integration) and a low power consumption, is a so-called MRAM (Magnetic Random Access Memory), for example, described by Wang et al., in IEEE Trans. Magn. 33 (1997), 4498, which has begun to draw much attention by recent remarkable improvements in the characteristics of TMR (Tunnel Magnetoresistance) materials.
An MRAM is a semiconductor memory which utilizes a magnetoresistance effect based on a spin dependent conducting phenomenon characteristic to a nano-magnetic substance, and is a non-volatile memory capable of retaining information without power supply from external.
Moreover, because of its simple structure, MRAM is easy to integrate into a large scale IC, and because of its recording of information based on spinning of magnetic moments, the number of rewritable frequencies is large, and also its access time is expected to become very fast as reported by R. Scheuerlein et al., in the ISSCC Digest of Technical Papers, pp. 128–129, Feb. 2000, where operability at 100 MHz was already reported.
Such a MRAM will be described more in detail. As illustrated in FIG. 14, a TMR element 10 which constitutes a memory cell in a memory element of MRAM includes a memory layer 2 in which magnetization is relatively easily rotated, and magnetization pinned layers 4, 6, laminated on a support substrate 9.
The magnetization pinned layer has two magnetization pinned layers of a first magnetization pinned layer 4 and a second magnetization pinned layer 6, and a conductive layer 5 is interposed therebetween for antiferromagnetically coupling these magnetic layers. As a memory layer 2 and magnetization pinned layers 4 and 6, a ferromagnetic material made of nickel, iron or cobalt, or an alloy thereof is used. Also, as a material of the conductive layer 5, any of ruthenium, copper, chrome, gold, silver or the like can be used. The second magnetization pinned layer 6 abuts an antiferromagnetic material layer 7. An exchange interaction occurring between these layers causes the second magnetization pinned layer 6 to have a strong unidirectional magnetic anisotropy. As a material of the antiferromagnetic material layer 7, a manganese alloys with such as iron, nickel, platinum, iridium, rhodium, or cobalt or nickel oxides may be used.
Further, between the memory layer 2 as the magnetic layer and the first magnetization pinned layer 4 there is sandwiched a tunnel barrier layer 3 which is an insulating body made of an oxide or nitride of aluminum, magnesium, silicon and the like, and functions to cut off a magnetic coupling between the memory layer 2 and the magnetization pinned layer 4 and also to pass a tunneling current therethrough. These magnetic layers and conductive layers are formed basically by a sputtering method, however, the tunnel barrier layer 3 can be obtained by oxidizing or nitriding a metal film deposited by sputtering. A top coat layer 1, which has a function to prevent mutual diffusion between the TMR element 10 and wiring to be connected thereto, to reduce a contact resistance, and to inhibit oxidization of the memory layer 2, can be made using such a material as Cu, Ta, TiN or the like. An underlayer electrode 8 is for connection with a switching element to be connected in series with the TMR element. This underlayer electrode 8 may serve also as an antiferromagnetic material layer 7.
In the memory cell constructed as described above, although information is read out by detecting changes in a tunneling current by the magnetoresistance effect to be described later, the effect thereof depends on relative orientations of magnetization in the memory layer and the magnetization pinned layer.
FIG. 15 is an enlarged perspective view showing a simplified portion of a general MRAM. Here, although a read-out circuit portion is omitted for simplification, there are included, for example, 9 pieces of memory cells, and mutually intersecting bit lines 11 and writing word lines 12. At each intersection therebetween, a TMR element 10 is disposed. Writing to the TRM element 10 is carried out in such a manner that by passing a current through the bit line 11 and the write word line 12, and by using a synthetic magnetic field resulting from respective magnetic fields generated therefrom, an orientation of magnetization in the memory layer 2 in the TMR element 10 disposed at each intersection of the bit line 11 and the write word line 12 is caused to rotate parallel or anti-parallel relative to that in the magnetization pinned layer.
FIG. 16 schematically illustrates a cross section of a memory cell, in which, for example, an n-type field effect transistor 19 for read out is disposed, comprising a gate insulation film 15, a gate electrode 16, a source region 17 and a drain region 18 which are formed in a p-type well region 14 formed in a p-type silicon semiconductor substrate 13, and above thereof there are disposed a write word line 12, a TMR element 10 and a bit line 11. To the source region 17, a sense line 21 is connected via a source electrode 20. The field effect type transistor 19 functions as a switching element for read-out, and a wiring 22 for reading which is wired out from between the word line 12 and the TMR element 10 is connected to the drain region 18 via a drain electrode 23. By way of example, the transistor 19 may be of any of n-type or p-type field effect transistors, and is not limited thereto, and various types of other switching elements such as diodes, bipolar transistors, MESFET (Metal Semiconductor Field Effect Transistors) and the like can be used.
FIG. 17 shows an equivalent circuit of an MRAM, which has, for example, 6 pieces of memory cells, mutually intersecting bit lines 11 and write word lines 12, and TMR elements 10 provided at each intersection of these write word lines together with a field effect transistor 19 for selection of an element for reading which is connected to the TMR element 10 and to a sense line 21. The sense line 21 which is connected to a sense amplifier 27 detects stored information. By way of example, numeral 24 in the drawing depicts a bidirectional write word line current drive circuit, and 25 depicts a bit line current drive circuit.
FIG. 18 is an asteroid curve showing write conditions to write to the MRAM, and indicates a threshold value for reversing the orientation of magnetization in the memory layer by a magnetic field HEA applied in the directions of the easy axis of magnetization and a magnetic field HHA applied in the directions of the difficult axis of magnetization. If a synthetic magnetic field vector is produced outside this asteroid curve, a reversal of the magnetic field occurs, however a synthetic magnetic field produced inside the asteroid curve does not cause the cell to be reversed from either one of its current bistable state to the other. Further, also in any cells other than at intersections of the word lines and the bit lines both passing through the currents, because a singular magnetic field generated either by the word line or the bit line is applied thereto, if a magnitude thereof exceeds a monodirectional reversal magnetic field Hk, the orientation of magnetization in any cells outside the aforementioned intersections may be reversed. Therefore, it is arranged to permit a selective writing to a selected cell only when the synthetic magnetic field falls in a grey-colored region in the drawing.
As described above, as to the MRAM, it is general that by use of two write lines of the bit line and the word line, and utilizing the asteroid magnetization reversal characteristics, only a designated cell is allowed selectively to write in to be effected by reversal of a magnetic spin. A synthetic magnetic field in a unit memory region is determined by a vector synthesis of a magnetic field applied in the direction of the easy axis of magnetization HEA and a magnetic field applied in the direction of the difficult axis of magnetization HHA. A current passing through the bit line applies a magnetic field in the direction of the easy axis of magnetization HEA to the cell, and a current passing through the word line applies a magnetic field in the direction of the difficult axis of magnetization HHA to the cell.
FIG. 19 illustrates operation of reading from MRAM. Here, a schematic diagram of a layer structure of a TMR element 10 is shown, in which the aforementioned magnetization pinned layer is depicted as a monolayer 26, and parts other than a memory layer 2 and a tunnel barrier layer 3 are omitted for simplification.
That is, as described above, writing of information is carried out by causing a synthetic magnetic field produced at the intersection between the bit lines 11 and the word lines 12 wired in a mesh to reverse the magnetic spin in the cell so as to store the information as “1” or “0”. Further, reading of information is carried out by utilizing a TMR effect which makes use of the magnetoresistance effect. Here, the TMR effect refers to such a phenomenon that a value of electrical resistance is changed depending on the orientation of the magnetic spin, and that by detecting a high resistance state if the magnetic spin is oriented anti-parallel (reverse direction) and a low resistance state if the magnetic spin is oriented parallel (same direction), information of “1” and “0” is detected. This reading is carried out by causing a read current (tunneling current) to pass through between the word line 12 and the bit line 11, and reading out an output therefrom in accordance with the aforementioned high resistance or low resistance to the sense line 21 via the field effect transistor 19 for reading.
As described above, although the MRAM is expected as a high speed and large capacity non-volatile memory, because of use of the magnetic material for retention of information, there is such a problem that information is erased or rewritten by the effect of an external magnetic field. This is because that a reversing magnetic field HSW in the directions of the easy axis of magnetization and the difficult axis of magnetization described with reference to FIG. 18 is small in a range of 20 to 200 Oe, though it depends on a material, and a few mA in terms of an electric current (R. H. Koch et al., Phys. Rev. Lett. 84, 5419 (2000), J. Z. Sun et al., 2001 8th Joint Magnetism and Magnetic Material). In addition, because a coercive force (Hc) when writing is in a range of approximately a few Oe to 10 Oe, if an internal leakage magnetic field greater than that resulting from an external magnetic field is applied, it sometimes becomes impossible selectively to write to a designated memory cell.
Therefore, as one step to actual application of the MRAM, it is ardently desired to establish countermeasures against external magnetic fields, that is, an effective magnetic shield structure for shielding the elements from external electromagnetic waves.
An environment in which an MRAM is packaged and used is mainly on a high density packaging substrate, and inside electronics equipment. Although it depends on the types of electronic equipment, by recent developments of high density packaging techniques, there are densely packaged a variety of semiconductor elements, communication elements, a micro-motor and the like on a high density packaging substrate, and also antenna elements, various mechanical parts, power source and the like are densely packaged inside the electronic equipment thereby constructing a unit of equipment.
A capability of a mixed or hybrid packaging as described above is one of the features of an MRAM as a non-volatile memory, however, because of its environment surrounding the MRAM in which various magnetic field components in a broad frequency range including dc, low frequencies to high frequencies are mixed and coexist, it is required, in order to ensure reliability of information retaining in MRAM, to improve the durability thereof against the external magnetic fields by developing a new packaging method and a new shield structure of the MRAM itself.
As to a magnitude of such external magnetic fields, for example, in a magnetic card such as a credit card or a bank cash card, it is specified to have durability against a magnetic field from 500 to 600 Oe. Therefore, in the field of the magnetic card, a magnetic material having a large coercive force such as a Co clad gamma-Fe2O3, Ba ferrite or the like are used in compliance therewith. Also, in the field of prepaid cards, it is necessary to have durability against magnetic fields from 350 to 600 Oe. Because the MRAM element is packaged inside electronic equipment, and is a device expected to be carried on, it needs to have enough durability against strong external magnetic fields equivalent to that of the magnetic cards, and in particular, because of the reason described above, a magnitude of an internal (leakage) magnetic field is required to be suppressed below 20 Oe, or preferably below 10 Oe.
As a magnetic shield structure of MRAM, it is proposed to use an insulating ferrite (MnZn ferrite and NiZn ferrite) layer as a passivation film of an MRAM element so as to provide a magnetic shielding characteristic (refer to U.S. Pat. No. 5,902,690, specification (column 5) and drawings (FIG. 1 & FIG. 3)). It is also proposed to attach a high permeability magnetic body such as Permalloy on the top and bottom of the package so as to provide a magnetic shielding effect and prevent penetration of a magnetic flux into the internal element (refer to U.S. Pat. No. 5,939,772, specification column 2, FIGS. 1, 2). Further, a structure of a shield lid made of a magnetic material such as soft iron or the like for cladding the element is disclosed (refer to Japanese Patent Application Publication No. 2001-250206, right-hand column on page 5, FIG. 6).
In order to prevent penetration of an external magnetic flux into the memory cell of MRAM, it is most important to clad the element with a magnetic material having a high permeability so as to provide a magnetic path thereby allowing no magnetic flux to penetrate any further.
However, when the passivation film of the element is formed from ferrite as disclosed in U.S. Pat. No. 5,902,690, because of a low magnetic saturation in the ferrite itself (for example, 0.2 to 0.5 tesla (T) in general ferrite materials), it is impossible completely to prevent penetration of external magnetic fields. Magnetic saturation in the ferrite itself is approximately 0.2 to 0.35 T in NiZn ferrite, and 0.35 to 0.47 T in MnZn ferrite, however, a magnitude of an external magnetic field penetrating into the MRAM element is as large as several hundreds Oe, thereby only with such a degree of saturation magnetization provided by the ferrite, a permeability becomes almost “1” due to magnetic saturation in the ferrite thereby disabling its function. Further, although a film thickness is not described in U.S. Pat. No. 5,902,690, because a thickness of a normal passivation film is about 0.1 μm or so at most, it is too thin to serve as a magnetic shield layer, thereby any substantial effect cannot be expected. Moreover, in the case if ferrite is to be used as the passivation film, because the ferrite is an oxide magnetic material, when it is deposited by sputtering, an oxygen defect tends to occur, thereby making it difficult to obtain a perfect ferrite to be used as the passivation film.
Further, in U.S. Pat. No. 5,939,772, a structure for sandwiching the package between the upper and the bottom Permalloy layers is disclosed. By use of the Permalloy, a higher shield performance than that of the ferrite passivation film is obtained. However, although the permeability of the Mu metal disclosed in U.S. Pat. No. 5,939,772 is very high to become μi=100,000 or so, the magnetic saturation thereof is very low to be 0.7 to 0.8 T, at which it will easily saturate to an external magnetic field consequently to become μ=1, therefore, there is a problem that in order to obtain a perfect magnetic shielding effect, a thickness of the shield layer must be increased considerably large. Therefore, as the structure for enabling to prevent penetration of magnetic fields of several hundreds Oe, in practice, it is not yet perfect as the magnetic shield layer in view of both drawbacks that the saturation magnetization thereof is too small and that the thickness thereof is too thin.
Still further, in Japanese Patent Application Publication No. 2001-250206, although a magnetic shield structure using soft iron or the like is disclosed, as this covers only the upper portion of the element, it is not perfect as the magnetic shield. Also, the magnetic characteristics thereof are not sufficient because that the saturation magnetization of the soft iron is 1.7 T and the permeability thereof is μi=300 or so. Therefore, even if a magnetic shield is fabricated using the structure disclosed in Japanese Patent Application Publication JP-A Laid-Open No. 2001-250206, it would be very difficult to completely prevent the penetration of external magnetic fields.
The present invention is contemplated to solve the aforementioned problems associated with the prior art, and to provide means for magnetically shielding MRAM elements sufficiently against large external magnetic fields, and also to guarantee reliable operation of the MRAM elements in an environment surrounding the MRAM elements which produces various magnetic fields.